In this kind of burner, there is conventionally known one in which the combustion plate part is constituted by: a burner frame in the shape of a picture frame; a metal-fiber knit (i.e., a metal knit formed of a heat resistant metal fiber) which is disposed to cover an opening enclosed by the burner frame; and a distribution plate which has formed therein a multiplicity of distribution holes and which sandwiches the metal-fiber knit between the burner frame and the distribution plate. Air-fuel mixture is thus arranged to be ejected through the distribution holes and the metal-fiber knit (see, for example, JP-A-2014-9839). Further, the flame rod is disposed in a manner to lie opposite to a portion of the opening enclosed by the burner frame, i.e., to lie opposite to a portion of the metal-fiber knit.
When the flame rod gets heated as a result of exposure to the flame, components such as aluminum and the like contained in the constituent material of the rod will be precipitated on the surface and are oxidized, and will be coated by an oxidizing film such as aluminum oxide and the like. In this manner, thanks to the oxidizing film the heat-resisting property of the flame rod can be secured. Although the oxidizing film has insulating properties, the oxidizing film will give rise to cracks, during burner combustion, due to expansion by heating of the flame rod. As a result, ions in the flame will come into contact, through the cracks, with the base material of the flame rod, thereby causing flame current to flow. However, it takes some time for the oxidizing film to give rise to cracks after ignition, and therefore the flame detection cannot be made during that period of time.